thermal/core: Add a thermal zone 'devdata' accessor

The thermal zone device structure is exposed to the different drivers
and obviously they access the internals while that should be
restricted to the core thermal code.

In order to self-encapsulate the thermal core code, we need to prevent
the drivers accessing directly the thermal zone structure and provide
accessor functions to deal with.

Provide an accessor to the 'devdata' structure and make use of it in
the different drivers.

No functional changes intended.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>